1. Field of the Invention
This invention relates to upgrading residual petroleum fractions by hydrothermal treatment and coking. More specifically, the invention relates to carefully limited hydrovisbreaking of such residua, fractionating to isolate catalytic cracking feedstock and bottoms, and delay coking of the bottoms to produce coke and additional cracking feedstocks.
2. Description of the Prior Art
In conventional processing of crude petroleum oil to recover fractions suitable as chargestock for catalytic cracking, the crude is first distilled at substantially atmospheric pressure. Gas and gasoline are recovered as overhead products, naphtha and perhaps a light gas oil are taken off as side streams, and the residual material is recovered from the bottom of the tower as atmospheric reduced crude. The residual fraction from the atmospheric tower is then passed to a vacuum distillation tower. The products of vacuum distillation include gas oil and a heavy residual fraction, described as vacuum reduced crude. The gas oil fraction is employed as catalytic cracking chargestock which can be a mixture of fractions otained by atmospheric and vacuum distillation. In general, it is a liquid distillate that boils in the range of about 500.degree.-1000.degree. F. (260.degree.-538.degree. C.).
To obtain additional catalytic cracking chargestock, it is also conventional to subject petroleum fractions heavier than gas oil, including residual fractions from atmospheric and vacuum distillation, to a thermal cracking procedure known as viscosity breaking, or, more commonly, as "visbreaking". This is essentially a single-pass, mild thermal cracking operation in which the heavy oil is passed at rather short residence time through a coil heated to a temperature in the range of about 850.degree.-950.degree. F. (454.degree.-510.degree. C.). The product is then separated to recover a gas oil cracking stock and a residue which is suitable for coking.
Coking is one of the refiner's major processes for converting residuals to lighter, more valuable stocks. Petroleum coke is the residue resulting from the thermal decomposition or pyrolysis of high-boiling hydrocarbons, particularly residues obtained from cracking or distillation of asphaltenic crude distillates. The hydrocarbons generally employed as feedstocks in the coking operation usually have an initial boiling point of about 700.degree. F. (380.degree. C.) or higher, an API gravity of about 0.degree.-20.degree., and a Conradson carbon residue content of about 5-40 weight percent.
The coking process is particularly advantageous when applied to refractory, aromatic feedstocks such as slurry decanted oils from catalytic cracking and tars from thermal cracking. In coking, the heavy aromatics in the resid are condensed to form coke, about 15-25 weight percent of the charge being used for coke making. The remaining material is cracked to naphtha and gas oil that can be charged to reforming and catalytic cracking.
Residual petroleum oil fractions, such as those fractions produced by atmospheric and vacuum crude distillation columns, are typically characterized as being undesirable as feedstocks for direct use in most refining processes. This undesirability is due primarily to the high content of contaminants, i.e., metals, sulfur, nitrogen, and Conradson carbon residue, that are present in said fractions.
Principal metal contaminants are nickel and vanadium, with iron and small amounts of copper also being sometimes present. Additionally, trace amounts of zinc and sodium are found in most feedstocks. As the great majority of these metals when present in crude oil are associated with very large hydrocarbon molecules, the heavier fractions produced by crude distillation contain substantially all of the metals present in the crude, such metals being particularly concentrated in the asphaltene residual fraction. The metal contaminants are typically large organo-metallic complexes such as metalloporphyrins and similar tetrapyrrole structures.
The residual fraction of single-stage atmospheric distillation or two-stage atmospheric/vacuum distillation also contains the bulk of the crude components which deposit as carbonaceous or coke-like material on cracking catalysts without substantial conversion. These are frequently referred to as "Conradson carbon" from the analytical technique of determining their concentration in petroleum fractions.
A process that combines hydrothermal treatment with coking is described in U.S. Pat. No. 1,995,005, wherein the residual oil is produced by thermally cracking a topped crude at temperatures of 800.degree.-1600.degree. F. (427.degree.-871.degree. C.) and pressures of atmospheric to 500 psig, removing the vaporous products, and delay coking the residual oil.
Cracking of higher boiling oils without a catalyst is discussed in U.S. Pat. No. 2,007,226. When higher boiling oils are heated to a cracking temperature, the cracked products include hydrocarbons relatively poor in hydrogen which tend to polymerize and form coke or solid or semi-solid pitches. However, the presence of hydrogen in concentrations sufficient to exert more than the characteristic minimum partial pressure tends to inhibit such polymerization and formation of coke and pitches. But, to be effective therefor, the hydrogen must be immediately present, with respect both to time and to place, as the constituent molecules of the higher boiling oil decompose or "crack".
A process for atmospheric distillation followed by vacuum distillation of a petroleum crude is described in U.S. Pat. No. 3,110,663. This process includes visbreaking a mixture of the atmospheric residuum and the vacuum residuum, visbreaking being defined as essentially a single-pass, mild thermal cracking operation in which the heavy oil is passed at rather short residence time through a coil heated to a temperature in the range of about 850.degree.-950.degree. F. (454.degree.-510.degree. C.).
An integrated hydrocarbon conversion process for converting a heavy hydrocarbon feedstock boiling above 650.degree. F. into products including gasoline, jet fuel, and coke is described in U.S. Pat. No. 3,891,538. This process comprises catalytically hydrodesulfurizing the feedstock to produce gasoline, jet fuel, a fraction boiling at 650.degree.-1000.degree. F. (343.degree.-538.degree. C.), and a fraction boiling above 1000.degree. F.; catalytically cracking the hydrodesulfurized material boiling at 650.degree.-1000.degree. F. to produce cracked products and a decant oil; coking the decant oil and the desulfurized product boiling above 1000.degree. F. to produce gasoline, coker gas oil, and the coke; and recycling the catalytic cracker gas cycle oil and coker gas oil to the hydrodesulfurizing operation.
It is pointed out in U.S. Pat. No. 4,005,006 that (1) hydrodesulfurization of catalytic residual oils can reduce their sulfur contents with relatively little hydrocracking if reaction temperatures are kept below about 790.degree. F. (421.degree. C.) and (2) it is advantageous to do so because catalytic hydrocracking reactions generally result in some production of naphtha which is relatively wasteful, for naphtha is easily and economically produced in the absence of added hydrogen by means of fluid catalytic cracking (FCC). Producing naphtha in an FCC process without added hydrogen saves the expense of hydrogen consumption and retains the olefins and aromatics in the naphtha product in an unsaturated state. Because olefins and aromatics are high octane number components, FCC naphtha generally exhibits higher research and motor octane values than does hydrocracked naphtha. This patent accordingly discloses thermal cracking or visbreaking of a residual oil, which may be nonhydrodesulfurized, to convert a portion thereof to middle distillates boiling at 350.degree.-650.degree. F. (177.degree.-343.degree. C.), with relatively small production of 350.degree. F..sup.- -( 177.degree. C..sup.-) naphtha and lighter material. Because thermal desulfurization occurs during visbreaking in proportion to the extent of conversion and regardless of whether or not the visbreaker feed oil is hydrodesulfurized, the relatively high conversion provides correspondingly high levels of desulfurization which is aided by, but does not require, the presence of added hydrogen. The visbreaking operation is performed at a preferred temperature of 790.degree.-950.degree. F. (421.degree.-510.degree. C.), a preferred pressure of 100-2500 psig (7-175 kg/cm.sup.2), and a preferred hydrogen feed rate of 500-2500 SCF per barrel (8.0-44.5 scm/100L), with a preferred oil residence time in the visbreaker of 0.3-3 hours.
The thermal treatment of residual oils for upgrading them to middle distillates boiling in the furnace oil, diesel fuel, and jet fuel range, in preference to the naphtha range, is described in U.S. Pat. No. 4,062,757.
An improved visbreaking process for residual oils is described in U.S. Pat. No. 4,062,757 in which the residual oil, with or without hydrogen, is passed upwardly through a packed bed of substantially stationary solids to produce improved middle distillate yield. Although it is commonly observed in conventional visbreaking processes that any increase in middle distillate yield is accompanied by disproportionate increase in naphtha yield caused by after cracking, it was found that enhanced production of middle distillates by this upflow process was achieved with both an enhanced yield of middle distillates and an enhanced product ratio of middle distillates to naphtha.
U.S. Pat. No. 4,324,645 describes the upgrading of residua by selectively removing CCR without undue hydrogen consumption by catalytic hydroprocessing to produce a particularly preferable feedstock for coking that gives more liquid yield and less coke make. The catalyst is one whose primary purpose is to limit hydrogen consumption for aromatics saturation and conversion of 1000.degree. F..sup.+ (538.degree. C..sup.+) material, i.e., reactions which selectively contribute to reduction of CCR. The majority of the sulfur is rejected with the coke so that prior sulfur removal during hydroprocessing is unnecessary if the major concern of refining is liquid product from coking rather than the quality of the coke make.
The combination of visbreaking with coking, as has been done in the prior art, tends to produce relatively large amounts of coke without using hydrogen and to be expensive with the use of hydrogen. This cost/benefit consideration is principally a matter of process economics. In terms of technical feasibility, catalytic hydrotreating of metal-containing resids, although expensive, is at least technically practical; however, with high metal-containing resids, the practicality of hyrdrotreating remains doubtful without new advances in technology. Another important consideration is that the addition of hydrogen to the large molecules as they are being thermally cracked during the visbreaking process seems to produce smaller molecules which are susceptible to losing the added hydrogen during subsequent processing. Moreover, the amount of hydrogen that is utilized by a feedstock during thermal treatment depends on the type of feed. If the feedstock is very low in hydrogen, for example, it needs more hydrogen in order to get gasoline plus distillates (G+D) as products in a reasonable amount. An Arab heavy resid is particularly known to be very low in hydrogen content so that unusually large amounts of hydrogen can be absorbed by this oil.
In terms of process economics, there is accordingly a need for a combined thermal treatment and coking process that minimizes costs and maximizes benefits.